doc: exclude spi_legacy.h from API docs
doc: exclude spi_legacy.h from API docs

spi.h and spi_legacy.h contain duplicate type and function names that
interfere with the doxygen-based API documentation.

fixes: #1425

Signed-off-by: David B. Kinder <c56c8b29908a5b2af1b63001268d73a0b7f972ba@intel.com>

GiulianoFranchetto/zephyr,zephyriot/zephyr,aceofall/zephyr-iotos,punitvara/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,fbsder/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,fbsder/zephyr,kraj/zephyr,Vudentz/zephyr,finikorg/zephyr,fbsder/zephyr,punitvara/zephyr,fbsder/zephyr,punitvara/zephyr,mbolivar/zephyr,zephyriot/zephyr,galak/zephyr,galak/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,zephyriot/zephyr,zephyrproject-rtos/zephyr,mbolivar/zephyr,nashif/zephyr,explora26/zephyr,finikorg/zephyr,galak/zephyr,aceofall/zephyr-iotos,explora26/zephyr,nashif/zephyr,mbolivar/zephyr,GiulianoFranchetto/zephyr,ldts/zephyr,explora26/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,zephyriot/zephyr,punitvara/zephyr,nashif/zephyr,mbolivar/zephyr,finikorg/zephyr,Vudentz/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,kraj/zephyr,kraj/zephyr,kraj/zephyr,mbolivar/zephyr,aceofall/zephyr-iotos,zephyriot/zephyr,ldts/zephyr,finikorg/zephyr,aceofall/zephyr-iotos,explora26/zephyr,aceofall/zephyr-iotos,punitvara/zephyr,Vudentz/zephyr,nashif/zephyr,fbsder/zephyr,galak/zephyr